1. Field of the Invention
The present invention relates to a thin-film piezoelectric resonator and a filter circuit that use longitudinal resonance in a width direction of a piezoelectric film.
2. Related Art
A surface acoustic wave (SAW) resonator is generally used as radio frequency (RF) and intermediate frequency (IF) filters in a mobile communication apparatus. However, since a resonant frequency of the SAW resonator is inversely proportional to a distance between comb electrodes, the distance between comb electrodes is reduced below 1 μm in a frequency domain exceeding 1 GHz. This imposes manufacturing limitations on increasing the resonant frequency. The SAW resonator, therefore, may not accommodate the increased frequency that is demanded to use in recent years.
Instead of the SAW resonator, a thin-film piezoelectric resonator that uses a thickness longitudinal oscillation mode of a piezoelectric thin film attracts attention. The thin-film piezoelectric resonator is also called as an FBAR (Film Bulk Acoustic Resonator) or a BAW (Bulk Acoustic Wave) element. In the thin-film piezoelectric resonator, a resonant frequency depends on a sound velocity and a film thickness of a piezoelectric body. Usually, a film thickness of 1 to 2 μm corresponds to a frequency of 2 GHz and a film thickness of 0.4 to 0.8 μm corresponds to a frequency of 5 GHz. It is possible to use a frequency as high as several tens GHz.
However, in the FBAR, the resonant frequency depends on thickness of the piezoelectric thin film. Thus, it is difficult to realize, as a single device, a resonator that copes with plural frequencies, that is, a so-called multiband resonator.
Besides, a thin-film piezoelectric resonator that uses a longitudinal resonance mode in a width direction of a piezoelectric thin film has been also proposed (see JP-A 7-147526 (Kokai) and Proceedings of The 13th International Conference on Solid-State Sensors, Actuators and Microsystems, Seoul, Korea, Jun. 5-9, 2005, pp. 2065-2068). This type of thin-film piezoelectric resonator is called as a contour mode acoustic resonator, a Lame mode acoustic resonator, or the like.
In the contour mode acoustic resonator, a resonant frequency depends on a sound velocity of a piezoelectric body and the width of a resonator. Thus, there is an advantage that it is possible to provide a multiband resonator as a single device. However, as in the SAW resonator, since the width of the resonator is inversely proportional to the resonant frequency, it is difficult, for example, to cope with a frequency in a GHz order or higher frequencies.